Double-strand breaks in DNA (DSBs) are among the most lethal lesions in the genome if they are not repaired or if their repair is executed incorrectly. Most cancer therapies, including radiation therapy, induces multiple DSBs to kill cancer cells but concurrent induction of DSBs in non-tumor cells can result in chromosome rearrangements that might be a source of therapy related tumors in treated patients. DSB repair takes place through two main pathways, homology-directed repair (HDR) and non-homologous end-joining (NHEJ), that are carefully regulated to avoid the formation of chromosomal aberrations. DSB repair regulation is of great importance to human health since errors in the choice of DSB repair pathway can incite gene rearrangements that promote cancer and faulty DSB repair often generates aberrant chromosomal structures that kill cells. This proposal is focused on the regulation of DSB repair by 53BP1, a DNA damage response factor that affects the choice between HDR and NHEJ, promoting NHEJ and inhibiting HDR. 53BP1 has recently attracted attention because of its involvement in the treatment of BRCA-deficient breast and ovarian cancers with PARP inhibitors (PARPi) that generate DSBs in S/G2. Brca1-deficient cells are sensitive to PARPi treatment because they fail to repair PARPi-induced DSBs by homologous recombination (HR) and accumulate aberrantly linked lethal chromosomes formed by NHEJ. However, when 53BP1 is absent, inappropriate DSB repair by NHEJ is diminished, HR appears to be restored, and the PARPi treatment is no longer effective. This proposal aims to elucidate the mechanisms by which 53BP1 controls the balance between NHEJ and HDR. We propose to use unique aspects of a telomere-based assay system developed in our laboratory to determine the mechanism and consequences of three functional aspects of 53BP1. In AIM 1, we will ask how the 53BP1-interacting factor Rif1 inhibits CtIP-dependent resection at DSBs and what the consequences are of this regulatory pathway for DSB repair, including in PARPi-treated Brca1-deficient cells. In AIM 2, we will use a second, newly-developed telomere-based system to determine how 53BP1 inhibits a 5' resection pathway that is independent of CtIP. Finally, in AIM 3, we will focus on the ability of 53BP1 to increase the mobility of chromatin at/near sites of DNA damage, an attribute we discovered in the context of telomere dysfunction. The mechanism by which 53BP1 acts to change chromatin movement will be determined and we will address to what extent this pathway affects the repair of genome-wide DSBs. The experiments are designed to gain insights into fundamental aspects of the regulation of DSB repair by 53BP1 with the ultimate objective to provide information valuable to the use of PARP inhibitors for the treatment of breast and ovarian cancer and other cancer therapies, including radiation therapy in which DSB repair is central to the treatment outcome.